1. Field of the Invention
The present invention relates to a signal processing apparatus, a signal processing method, and a computer program, and, more particularly to a signal processing apparatus, a signal processing method, and a computer program that make it possible to prevent, in applying signal processing to an inputted stream and outputting the stream, a mismatch between a type of a codec used for encode of the inputted stream and a type of a codec used for encode of the stream to be outputted.
2. Description of the Related Art
As one of functions realized by optical disks of a Blu-ray (registered trademark) Disc standard and the like, there is an interactive function. According to the interactive function, it is possible to popup-display operation menus over a video during the play of contents such as a movie. A user can perform various kinds of operations from the operation menus by, for example, operating a remote controller to move a cursor.
At timing when the cursor displayed on the operation menus is moved or when an item selected by the cursor is determined, audio (effect audio) corresponding to operation of the user is outputted together with audio of the movie being played. When audio is outputted by an external apparatus such as an AV amplifier connected to a player for the optical disk, such output of the audio of the contents combined with the other audio is performed by applying signal processing in the player to an audio stream read out from the optical disk and reproducing an audio stream obtained by the signal processing using the AV amplifier.
When the signal processing is applied to the audio stream read out from the optical disk, usually, the audio stream is encoded by a predetermined codec and recorded on the optical disk in a compressed state. Thus, in the player, the audio stream is once decoded and the signal processing is applied to the uncompressed audio stream obtained by the decode.
The audio stream subjected to the signal processing by the player is outputted in an uncompressed state as a stream of linear PCM (Pulse Code Modulation) or the like or re-encoded by an arbitrary codec and outputted in a compressed state. An audio stream that does not need to be subjected to the signal processing is outputted to the AV amplifier while kept in the compressed state as read out from the optical disk.
JP-A-2005-20242 discloses a technique for causing one player to play contents recorded on plural DVDs and allowing plural users to simultaneously view the different contents.
When the audio stream subjected to the signal processing is outputted from the player in the uncompressed state, the audio stream after decode has a large amount of information compared with the audio stream before decode and more bands are necessary for a transmission line between the player and the AV amplifier. Thus, the entire audio stream may not be able to be transmitted because of the lack of bands of the transmission line.
For example, in an S/PDIF I/F (IEC60958), which is one of digital audio I/Fs, an uncompressed audio stream of linear PCM for two channels can only be transmitted at the maximum.
Therefore, when an audio stream recorded on the optical disk is an audio stream of a multi-channel equal to or more than three channels, in order to transmit the audio stream subjected to the signal processing in the uncompressed state through the S/PDIF I/F, it is necessary to discard (not to transmit) a stream of a channel that may not be able to be transmitted or down-mix the audio stream to a stream of two channels for transmitting the stream.
In this case, unlike the case in which the audio stream recorded on the optical disk is directly outputted to the AV amplifier without being subjected to the signal processing, the user may be unable to effectively enjoy audio recorded in contents according to an intention of a producer.
When an audio stream subjected to the signal processing is re-encoded by an arbitrary codec and outputted in the compressed state, as far as the codec used in the re-encode is the arbitrary one, a mismatch of codecs may occur. In other words, audio stream encoded by a codec of a type different from that of a codec used for encode of the audio stream recorded on the optical disk may be outputted from the player.
For example, assuming that an audio stream encoded by a codec A is recorded on the optical disk and the audio stream is decoded and subjected to the signal processing, and, then re-encoded by a codec B and outputted, if the user selects an AV amplifier having a decoder applicable to only decode in the codec A as an apparatus connected to the player, an audio stream directly outputted from the player without being subjected to the signal processing can be reproduced but an audio stream re-encoded in the player may not be able to be reproduced.
Even when a type of a codec used for encode of an audio stream recorded on the optical disk and a type of a codec used in re-encode happen to be the same, when the re-encode is performed in accordance with parameters different from encode parameters used for the encode of the audio stream recorded on the optical disk, a substantial difference in audio effect or the like occurs between audio outputted by directly decoding the audio stream recorded on the optical disk and audio outputted by decoding the re-encoded audio stream. The encode parameters include, for example, information (channel assignment, ACMOD, and AMODE) related to the number of channels and a structure of a surround speaker.
For example, in the case that an audio stream recorded on the optical disk is an audio stream of a multi-channel of three channels, when the audio stream read out from the optical disk is directly outputted to the AV amplifier without being subjected to the signal processing, the AV amplifier regards the audio stream supplied thereto as an audio signal of three channels and applies surround processing to the audio stream to drive a speaker.
On the other hand, when encode parameters are set so as to encode the audio stream recorded on the optical disk as an audio stream of 5.1 channels different from the number of channels of the audio stream recorded on the optical disk and the audio stream subjected the signal processing is re-encoded, regardless of the fact that the audio stream as a basis of the re-encode is only of three channels, the audio stream is outputted as an audio stream of 5.1 channels from the player. The AV amplifier that receives the audio stream applies, considering that the audio stream of 5.1 channels is supplied, the surround processing to the audio stream to drive the speaker. In other words, the AV amplifier processes the audio stream for 2.1 channels as a silent signal is supplied thereto.
In the former case and the latter case, different kinds of processings are performed by the AV amplifier. Thus, even if the original audio stream is the same, as a result of the processings, audio to be outputted and audio effect are substantially affected.
When the transmission line and the AV amplifier are selected assuming that an audio stream read out from the optical disk is always directly outputted from the player (while keeping the compressed state) without being subjected to the signal processing, the situation described above may occur.